or anguli oris. 16, The depressor labii inferioris. 18, The
sterno-hyoideus. 19, The platysma-myodes. 20, The superior belly of the
omo-hyoideus. 21, The sterno-cleido mastoideus. 20, The scalenus medius.
23, The inferior belly of the omo-hyoideus. 24, The trapezius.[5]

_Practical Explanation._ The muscle 1, 1, elevates the eyebrows. The
muscle 2 closes the eye. The muscle 6 elevates the upper lip. The muscles
7, 8, 9, elevate the angle of the mouth. The muscle 10 brings the teeth
together when eating. The muscle 11 depresses the upper lip. The muscle
13 closes the mouth. The muscle 15 depresses the angle of the mouth. The
muscle 16 draws down the lower lip. The muscles 18, 19, 20, 23, depress
the lower jaw and larynx and elevate the sternum. The muscle 21, when
both sides contract, draws the head forward, or elevates the sternum;
when only one contracts, the face is turned one side toward the opposite
shoulder. The muscles 18, 19, 20, 21, 22, 23, 24, aid in respiration.]

[5] In the plates illustrating the muscular system, the names of such
muscles are given as are referred to in the paragraph
"Practical Explanation." These names need not be committed to
memory. If a pupil wishes to acquire a knowledge of the general
attachment of the muscles represented in the plates, he can do
so by _comparing_ the muscular plate with that of the skeleton,
(fig. 28.)

_Observation._ When we are sick, and cannot take food, the body is
sustained by absorption of the fat. The removal of it into the blood
causes the sunken cheek, hollow eye, and prominent appearance of the
bones after a severe illness.

158. The number of muscles in the human body is more than five
hundred; in general, they form about the skeleton two layers, and are
distinguished into superficial and deep-seated muscles. Some of the
muscles are voluntary in their motions, or act under the government of
the will, as those which move the fingers, limbs, and trunk; while
others are involuntary, or act under the impression of their proper
stimulants, without the control of the individual, as the heart.

_Observations._ 1st. The abdominal muscles are expiratory, and the
chief agents for expelling the residuum from the rectum, the bile from
the gall bladder, the contents of the stomach and bowels when
vomiting, and the mucus and irritating substances from the bronchial
tubes, trachea, and nasal passages by coughing and sneezin